1. Field of the Invention
Embodiments of the present invention relate, in general, to systems and methods for reducing jitter associated with disk drive back electromotive force zero cross signal error and particularly to identifying and reducing the pair pole asymmetry contribution to jitter.
2. Relevant Background
Hard disk drives remain popular as long term and high volume storage devices for personal computers. Such drives typically include a plurality of rigid disks having coatings capable of being magnetized. Data can be written to these disks by magnetizing successive data cells defined along concentric data tracks of the disks. Because of the rigidity of the disks, the further inclusion of a servo system that is capable of precise track following permits the data tracks to be very closely spaced so that a hard disk drive can store a very large amount of data in a very small volume.
Modern disk drives use brushless dc motors (“BLDC”), also called spindle motors, to rotate the disks at the desired operating rotational speed. These motors generally have a stationary stator containing a plurality of phases, each phase having one or more sets of electrical windings or coils that are wrapped around teeth within the stator with the sets of windings in each phase being electrically connected in series. Each phase is generally connected with the other phases so that all phases share a common center tap as in a “Y” or “star” configuration, without a real center tap or as in a “Δ” configuration, or the like. The motors also have a rotor capable of rotation that contains a plurality of permanent magnet segments.
FIG. 1A shows a block diagram of a speed control circuit for a disk drive as is known in the prior art. The speed control circuit consists of a commutation circuit 10, a back electromotive force (“BEMF” or “back EMF”) detection circuit 12, a spindle speed control unit 18, a rotor 14, and a stator 16 with stator coils A, B, and C. It is well known in the art that a disk drive is manufactured by combining the block diagram of the speed control circuit in FIG. 1A with read/write heads, head motors, magnetic media, a rotor, and a disk drive housing. The rotor, which is shown in FIG. 1B, rotates responsive to coils A, B, and C being energized in a standard sequence, such as in bipolar operation. In such an operation (also called a two phase ON motor), the commutation circuit orients the magnetic flux in the motor so as to achieve peak efficiency. The spindle speed control 18 regulates the amplitude of the magnetic flux to either accelerate or decelerate the motor. The commutation circuit 10 also enables the BEMF of the floating coil to be buffered so that the BEMF detection circuit 12 can generate a zero cross signal from the buffered BEMF signal. According to this example, the zero cross signal is used by the commutation circuit 10 to determine the position of the rotor 14 relative to the stator coils A, B, and C and is used to control the rotational velocity of the rotor. The technique described above is a rough way of commuting the BLDC motor.
A zero cross is associated with the passing of a pair pole. FIG. 1B shows an expanded view of the rotor 14. In this particular example, there are 4 pair poles (north south pairs). According to one technique, each north pole represents a zero cross 15, thus for this example there are 4 zero crosses that are theoretically spaced at 90 degree intervals. More recently, the electrical period can be divided into more steps (typically 24, 48, or 96) in order to have a finer position control for magnetic flux. Furthermore, and according to other techniques known in the art, the current injected into coils A, B, and C varies in a sinusoidal waveform in phase with sinusoidal waveform of the BEMF signal in order to have a constant mutual torque with minimal torque variance.
Position, and therefore speed, of a disk drive platter is commonly determined by detecting the back EMF generated between one phase and the center tap (real or reconstructed). For example, it is typical for a disk drive motor to have six pair poles so that each pole-pair interaction theoretically signifies ⅙th of a motor rotation. However, in practice, it is difficult during manufacturing to accurately position the poles. In particular, the rotor may not be perfectly formed. Even if the rotor is perfectly formed, mechanical and electrical differences in the windings of the various coils could result in effective positional differences among the poles.
As described above, the disks are mechanically mounted to the rotor and rotated by the energizing of the coil windings within a phase in the stator to induce magnetic fields that interact with the permanent magnet segments in the rotor to cause the rotor and the disk(s), to rotate in the desired direction at the desired operating speed. The selective energizing of phases in the stator in a predetermined sequence is known as commutation of the motor, which simply involves providing a series of timed commutation steps wherein energy is imparted to the motor to cause it to rotate. The time between each commutation step is called a commutation period, and the number of commutation periods within one mechanical revolution of the motor is a function of the number of phases and the total number of north and south poles in the rotor magnet segments. For example, a three phase, twelve pole motor would have 36 commutation steps and 36 corresponding commutation periods in each mechanical revolution of the motor, with each commutation step occurring at a unique mechanical alignment of a winding and a pole, where a switch is made from one configuration or commutation step to the next.
As understood in the art, a commutation step involves supplying the motor with current to one phase, sinking current from another phase, and holding a third phase at a high impedance so that it remains un-energized. A commutation cycle is defined as a complete sequence of commutation steps such that every phase has, in turn, sourced current, sunk current, and remained un-energized current. The proper sequencing and timing of commutation steps will cause the motor to rotate in the desired direction at the desired speed, and generally there will be a plurality of commutation cycles for each mechanical revolution of the motor. The sequencing and timing of commutation steps are supplied to the motor by commutation circuitry to cause the motor to rotate at the desired operating speed.
Typically, modern disk drives have a back EMF sense circuit that measures the back EMF generated on the un-energized phase held at high impedance, compares this voltage to the voltage on the center tap (real or artificially generated), and generates a signal at a zero crossing of the voltages, that is, when the back EMF voltage changes polarity with respect to the voltage on the center tap. The point at which the zero crossing occurred is then used as a reference for the generation of the timing of the next commutation pulse, as well as a reference to indicate the position and relative speed of the motor. A zero crossing detector literally detects the transition of a signal waveform, ideally providing at narrow pulse that coincides exactly with the zero voltage condition. Note that when a sinusoidal driving mode is used, none of the phases are un-energized except shortly before the expected zero cross of the BEMF on one phase. This phase is put into a floating mode (un-energized) in order to detect/measure the BEMF. It is also very common in sinusoidal driving mode not to use all of the zero crossings but rather a limited set of zero crossings.
Simply stated, BEMF is voltage across the coil. BEMF is proportional to the magnet flux variation created by the rotor. Recall that the rotor comprises a permanent magnet that consists of a succession of north and south poles (a north and south pole comprising a pair pole). If a north pole is approaching (increasing magnetic flux), the coil in the BEMF is positive, and if a north pole is moving away (decreasing magnetic flux) from the coil, the BEMF is negative. Similarly, decreasing magnetic flux can mean a south pole approaching the coil. Because the BEMF is proportional to the derivate of the flux (and not the flux itself), the BEMF is also proportional to the spindle speed. Similarly, the number of pair poles associated with a motor corresponds directly to the ratio between the mechanical turn of the rotor and the electrical frequency. Thus, for example, a motor having four pair poles has four electrical periods (BEMFs) per mechanical turn of the rotor.
At the time the north pole is the most aligned with a pair of coils, the magnetic flux is changing from increasing to decreasing and so the BEMF is zero at that exact position (independent of the spindle speed); this point can be called zero cross with negative slope (increasing to decreasing flux). Another zero BEMF point is reached for the south pole; this one can be called zero cross with positive slope (decreasing to increasing flux). Because the zero crossings are independent of the spindle speed, they provide a precise yet independent means to know the rotor position.
It will be recognized by those of skill in the art that slight variations exist in the operating speed of a spindle motor during operation and that it becomes increasingly important to control and minimize these variations to ensure proper alignment of the transducer head and the sector before data is stored or retrieved from a disk.
One significant source of variation in the operating speed of a disk comes from manufacturing tolerances within the spindle motor itself. These manufacturing tolerances may be due to variations in bearings and bearing surfaces, variations in field strength of permanent magnets in the spindle motor, variations in resistance and inductance in phase windings in the spindle motor, and, most importantly, the variation in the spacing of the magnetic poles and the slots between windings. These variations can significantly affect the timing of the zero crossings, which in turn will affect the timing of the subsequent commutation steps and will affect the ability of the speed control circuitry to maintain the rotation of the disks at a constant speed.
Further, before data may be stored on a particular sector in a data track, the transducer head must first locate the correct data track and detect the boundary of the correct sector as it passes under the transducer head. This means the variations in the operating speed of the disks typically requires the use of a larger timing buffer to prevent data corruption than would be necessary if the motor were running at near zero speed variation.
A problem in many prior art hard disk drives that use such back EMF sensing and commutation timing methods is the use of the measured electrical period between two consecutive zero crosses to determine the timing of subsequent commutation steps (extrapolation method). Because of the variation associated with the timing of the zero crossings due to manufacturing tolerances in the spindle motor, significant error can exist in the placement of commutation steps based on the previous zero crossing period, which causes further motor speed variations due to misinterpretation by the spindle speed regulator or controller.
A related problem in many such prior art hard disk drives follows from the use of zero crossings to indicate the motor speed and position; because of the variations in zero crossings due to manufacturing tolerances within the spindle motor, the motor speed control may provide unnecessary compensation of the motor speed at subsequent commutation steps. For example, if, as a result of manufacturing tolerances, a zero crossing occurs at a point in time that indicates the motor is running too fast, when in fact the motor is running at the correct speed, the speed control circuitry may unnecessarily cause the motor to slow down by reducing the supplied current. It has been observed that a certain amount of this error is systematic for each mechanical rotation of the disks in a motor; a motor running through each mechanical revolution may be sped up or slowed down at the same steps based on these manufacturing variations while maintaining an overall average constant speed.
Some drive systems are operated in a linear mode with smoothly varying power continuously applied to the motor coils. Linear mode operation is favorable to detecting clean back EMF signals. However, under certain conditions, such as upon commutation of the drive between coil pairs of the motor, noise occurs that may give an inaccurate signal. One technique known in the art to compensate for this noise is to have, in addition to a period counter (an up counter) for measuring time between reference (e.g., zero) crossings of the back EMF, other counters including a delay counter and a mask counter to help minimize torque ripple and to avoid inaccurate signals. Such techniques are successful, particularly in linear mode operation, but nonetheless rely on an accurate zero cross measurement.
In addition to the linear mode, another way of operating a drive system is in a nonlinear mode with discontinuities in application of drive current to the coils. Pulse width modulation (“PWM”) is a known technology for operating drives in a nonlinear mode. In PWM operation, the power to the coils follows the same overall pattern as in the linear mode but is repeatedly chopped with brief segments of on-time alternating with brief segments of off-time. For example, there can be PWM operation with on-off cycles repeated at a frequency of between 25 kHz to 100 kHz. In a typical on-off cycle, lasting for about 20 us, there may be 14 us of on time followed by 6 us of off time, then a repeat of such a cycle would follow.
PWM operation has the attraction of lower power dissipation in electronic drivers. However, the switching in the PWM mode causes switching noise and raises the probability of an inaccurate BEMF signal despite presently known measures such as the above-mentioned mask counter and delay counter.
A significant challenge in providing consistent disk motor control is reducing spindle vibrations or jitter. “Jitter” is a term sometimes used in the motor control art to characterize conditions of instability where precise detection of zero crossings and precise switching of drive current from phase to phase are not precise. Spindle jitter has at least three components; 1) electrical noise, 2) pair pole asymmetry, and 3) real speed variation. The inherent nature of PWM operation makes it difficult to achieve low jitter in brushless, sensorless motor drives because of modulation or chopping that occurs near a zero crossing.
All of the aforementioned techniques for precise motor control rely on an accurate and reliable measurement of zero crossings. The aggressiveness of the spindle speed controller is limited by the amount of jitter. Thus, a reduction in jitter can allow the spindle speed controller to be more aggressive resulting in better measurement precision and spindle speed bandwidth.